In an image display device, for example, a liquid crystal display, a large number of pixels are arranged in a matrix and the light intensity is controlled for every pixel in accordance with the image information to be displayed so as to display an image.
This same is true for an organic EL display etc. An organic EL display is a so-called self-light emitting type display having a light emitting element in each pixel circuit and has the advantages that the viewability of the image is higher in comparison with a liquid crystal display, a backlight is unnecessary, the response speed is high, etc.
Further, it greatly differs from a liquid crystal display etc. in the point that the gradations of the color generation are obtained by controlling the luminance of each light emitting element by the value of the current flowing through it, that is, each light emitting element is a current controlled type.
An organic EL display, in the same way as a liquid crystal display, may be driven by a simple matrix and an active matrix system. While the former has a simple structure, it has the problem that realization of a large sized and high definition display is difficult. For this reason, much effort is being devoted to development of the active matrix system of controlling the current flowing through the light emitting element inside each pixel circuit by an active element provided inside the pixel circuit, generally, a TFT (thin film transistor).
FIG. 1 is a block diagram of the configuration of a general organic EL display device.
This display device 1 has, as shown in FIG. 1, a pixel array portion 2 comprised of pixel circuits (PXLC) 2a arranged in an m×n matrix, a horizontal selector (HSEL) 3, a write scanner (WSCN) 4, data lines DTL1 to DTLn selected by the horizontal selector 3 and supplied with a data signal in accordance with the luminance information, and scanning lines WSL1 to WSLm selectively driven by the write scanner 4.
Note that the horizontal selector 3 and the write scanner 4 are sometimes formed around the pixels by MOSICs etc. when formed on polycrystalline silicon.
FIG. 2 is a circuit diagram of an example of the configuration of a pixel circuit 2a of FIG. 1 (refer to for example U.S. Pat. No. 5,684,365 and Patent Publication 2: Japanese Unexamined Patent Publication (Kokai) No. 8-234683).
The pixel circuit of FIG. 2 has the simplest circuit configuration among the large number of proposed circuits and is a so-called two-transistor drive type circuit.
The pixel circuit 2a of FIG. 2 has a p-channel thin film FET (hereinafter, referred to as TFT) 11 and TFT 12, a capacitor C11, and a light emitting element 13 comprised of an organic EL element (OLED). Further, in FIG. 2, DTL indicates a data line, and WSL indicates a scanning line.
An organic EL element has a rectification property in many cases, so sometimes is referred to as an OLED (organic light emitting diode). The symbol of a diode is used as the light emitting element in FIG. 2 and the other figures, but a rectification property is not always required for an OLED in the following explanation.
In FIG. 2, a source of the TFT 11 is connected to a power source potential VCC, and a cathode of the light emitting element 13 is connected to a ground potential GND. The operation of the pixel circuit 2a of FIG. 2 is as follows.
<Step ST1>:
When the scanning line WSL is made a selected state (low level here) and a write potential Vdata is supplied to the data line DTL, the TFT 12 becomes conductive, the capacitor C11 is charged or discharged, and the gate potential of the TFT 11 becomes Vdata.
<Step ST2>:
When the scanning line WSL is made a non-selected state (high level here), the data line DTL and the TFT 11 are electrically separated, but the gate potential of the TFT 11 is held stably by the capacitor C11.
<Step ST3>:
The current flowing through the TFT 11 and the light emitting element 13 becomes a value in accordance with a gate-source voltage Vgs of the TFT 11, while the light emitting element 13 is continuously emitting light with a luminance in accordance with the current value.
As in the above step ST1, the operation of selecting the scanning line WSL and transmitting the luminance information given to the data line to the inside of a pixel will be referred to as “writing” below.
As explained above, in the pixel circuit 2a of FIG. 2, if once the Vdata is written, the light emitting element 13 continues to emit light with a constant luminance in the period up to the next rewrite operation.
As explained above, in the pixel circuit 2a, by changing a gate application voltage of the drive transistor constituted by the TFT 11, the value of the current flowing through the EL light emitting element 13 is controlled.
At this time, the source of the p-channel drive transistor is connected to the power source potential Vcc, so this TFT 11 is always operating in a saturated region. Accordingly, it becomes a constant current source having a value shown in the following equation 1.Ids=½·μ(W/L)Cox(Vgs−|Vth|)2  (1)
Here, p indicates the mobility of a carrier, Cox indicates a gate capacitance per unit area, W indicates a gate width, L indicates a gate length, Vgs indicates the gate-source voltage of the TFT 11, and Vth indicates the threshold value of the TFT 11.
In a simple matrix type image display device, each light emitting element emits light only at a selected instant, while in an active matrix, as explained above, each light emitting element continues emitting light even after the end of the write operation. Therefore, it becomes advantageous in especially a large sized and high definition display in the point that the peak luminance and peak current of each light emitting element can be lowered in comparison with a simple matrix.
FIG. 3 is a view of the change along with elapse of the current-voltage (I-V) characteristic of an organic EL element. In FIG. 3, the curve shown by the solid line indicates the characteristic in the initial state, while the curve shown by the broken line indicates the characteristic after change along with elapse.
In general, the I-V characteristic of an organic EL element ends up deteriorating along with elapse as shown in FIG. 3.
However, since the two-transistor drive system of FIG. 2 is a constant current drive system, a constant current is continuously supplied to the organic EL element as explained above. Even if the I-V characteristic of the organic EL element deteriorates, the luminance of the emitted light will not change along with elapse.
The pixel circuit 2a of FIG. 2 is comprised of p-channel TFTs, but if it were possible to configure it by n-channel TFTs, it would be possible to use an amorphous silicon (a-Si) process in the past in the fabrication of the TFTs. This would enable a reduction in the cost of TFT boards.
Next, consider a pixel circuit replacing the transistors with n-channel TFTs.
FIG. 4 is a circuit diagram of a pixel circuit replacing the p-channel TFTs of the circuit of FIG. 2 with n-channel TFTs.
The pixel circuit 2b of FIG. 4 has an n-channel TFT 21 and TFT 22, a capacitor C21, and a light emitting element 23 comprised of an organic EL element (OLED). Further, in FIG. 4, DTL indicates a data line, and WSL indicates a scanning line.
In the pixel circuit 2b, the drain side of the drive transistor constituted by the TFT 21 is connected to the power source potential Vcc, and the source is connected to the anode of the EL element 23, whereby a source-follower circuit is formed.
FIG. 5 is a view of the operating point of a drive transistor constituted by the TFT 21 and an EL element 23 in the initial state. In FIG. 5, the abscissa indicates the drain-source voltage Vds of the TFT 21, while the ordinate indicates the drain-source current Ids.
As shown in FIG. 5, the source voltage is determined by the operating point of the drive transistor constituted by the TFT 21 and the EL light emitting element 23. The voltage differs in value depending on the gate voltage.
This TFT 21 is driven in the saturated region, so a current Ids of the value of the above equation 1 is supplied for the Vgs for the source voltage of the operating point.
However, here too, similarly, the I-V characteristic of the EL element ends up deteriorating along with elapse. As shown in FIG. 6, the operating point ends up fluctuating due to this change along with elapse. The source voltage fluctuates even if supplying the same gate voltage.
Due to this, the gate-source voltage Vgs of the drive transistor constituted by the TFT 21 ends up changing and the value of the current flowing fluctuates. The value of the current flowing through the EL light emitting element 23 simultaneously changes, so if the I-V characteristic of the EL light emitting element 23 deteriorates, the luminance of the emitted light will end up changing along with elapse in the source-follower circuit of FIG. 4.
Further, as shown in FIG. 7, a circuit configuration where the source of the drive transistor constituted by the n-channel TFT 31 is connected to the ground potential GND, the drain is connected to the cathode of the EL element 33, and-the anode of the EL light emitting element 33 is connected to the power source potential Vcc may be considered.
With this system, in the same way as when driven by the p-channel TFT of FIG. 2, the potential of the source is fixed, the drive transistor constituted by the TFT 31 operates as a constant current source, and a change in the luminance due to deterioration of the I-V characteristic of the EL light emitting element 33 can be prevented.
With this system, however, the drive transistor has to be connected to the cathode side of the EL light emitting element. This cathodic connection requires development of new anode-cathode electrodes. This is considered extremely difficult with the current level of technology.
From the above, in the past systems, no organic EL element using a n-channel transistor free of change in luminance has been developed.